Protein synthesis is a fundamental biological process that underlies the development of polypeptide therapeutics, vaccines, diagnostics, and industrial enzymes. With the advent of recombinant DNA (rDNA) technology, it has become possible to harness the catalytic machinery of the cell to produce a desired protein. This can be achieved within the cellular environment or in vitro using lysates derived from cells.
Because only twenty amino acids are naturally incorporated into proteins, limitations to the production of a desired protein exist. For example, a peptide that is potentially useful as a therapeutic agent may be quickly degraded or otherwise inactivated upon administration to a patient as a result of proteases present within the patient. Likewise, infectious agents such as bacteria or viruses are more likely to develop resistance against peptides that contain only naturally occurring amino acids. This occurs because enzymes that are produced by the bacteria or virus that can inactivate a peptide drug are more likely to inactivate a peptide containing naturally occurring amino acids as opposed to a peptide containing non-native amino acids. Such limitations become even more apparent when compared with small organic molecule synthesis, in which any structural change can be made to influence functional properties of the compound. As a result, proteins containing non-native amino acids are becoming more auspicious for therapeutic uses. Furthermore, peptides containing non-native amino acids are extremely useful for non-therapeutic research purposes, such as uses relevant to the structural and functional probing of proteins, construction of peptide libraries for combinatorial chemistry, and proteomic studies.
Although the twenty naturally occurring amino acids can be modified by post-translational modification, expanding the genetic code to include additional non-native amino acids with novel biological, chemical, or physical properties will increase the utility of the protein containing such novel non-native amino acids. Protein properties may include the size, acidity, nucleophilicity, hydrogen-bonding, or hydrophobicity of the protein.
Different strategies have been utilized to synthesize peptides containing non-native amino acids. Synthetic peptide chemistry has been used routinely for this purpose. See, e.g., Eckert et al., Cell 99:103-15 (1999). However, routine solid-phase peptide synthesis is generally limited to small peptides with less than 100 residues. With the recent development of enzymatic ligation and native chemical ligation of peptide fragments, it is possible to make larger proteins. However, these methods are not easily scaled. See, e.g., Dawson and Kent, Annu Rev. Biochem. 69:923 (2000).
In vivo translation using living cells is widely used for the efficient synthesis and post-translational modification of proteins from a genetically encoded natural or recombinant DNA sequence. However, folding may be inefficient if the protein is expressed in inclusion bodies. Most importantly, such methods are more difficult for the selective incorporation of multiple non-native amino acids, or to control the post-translational modification process.
In vitro, or cell-free, protein synthesis offers several advantages over conventional in vivo protein expression methods. Cell-free systems can direct most, if not all, of the metabolic resources of the cell towards the exclusive production of one protein. Moreover, the lack of a cell wall and membrane components in vitro is advantageous since it allows for control of the synthesis environment. For example, tRNA levels can be changed to reflect the codon usage of genes being expressed. The redox potential, pH, or ionic strength can also be altered with greater flexibility than with in vivo protein synthesis because concerns of cell growth or viability do not exist. Furthermore, direct recovery of purified, properly folded protein products can be easily achieved.
The productivity of cell-free systems has improved over 2-orders of magnitude in recent years, from about 5 μg/ml-hr to about 500 μg/ml-hr. Such improvements have made in vitro protein synthesis a practical technique for laboratory-scale research and provides a platform technology for high-throughput protein expression. It further indicates the feasibility for using cell-free technologies as an alternative means to in vivo large-scale, commercial production of protein pharmaceuticals.
The incorporation of non-native amino acids into proteins remains a challenge with both in vivo and in vitro protein synthesis systems. A major hurdle in this field of endeavor is promoting recognition of an aminoacyl-tRNA synthetase with a non-native amino acid. An aminoacyl-tRNA synthetase is an enzyme that catalyzes the bond of a specific amino acid to its cognate tRNA molecule. In most cases, each naturally occurring amino acid has one specific aminoacyl-tRNA synthetase that will aminoacylate that amino acid to its proper tRNA molecule, which is known as tRNA charging. There exists relatively few aminoacyl-tRNA synthetases considering the fact that the degeneracy of the genetic code allows amino acids to be charged to more than one kind of isoaccepting sense tRNA molecule. Thus, the success of incorporating non-native amino acids into proteins depends on the recognition of the non-native amino acid by aminoacyl-tRNA synthetases, which in general requires high selectively to insure the fidelity of protein translation. The fidelity of aminoacylation is maintained both at the level of substrate discrimination and proofreading of both non-cognate intermediates and protein products.
One strategy has been to incorporate non-native amino acids into proteins using aminoacyl-tRNA synthetases that cannot discriminate between non-native amino acids that are structurally similar to their natural counterparts due to lack of proofreading mechanisms. Because the proofreading activity of the aminoacyl-tRNA synthetase has been disabled, structural analogs of natural amino acids that have been misactivated may escape the editing functions of the synthetase, and be incorporated into the growing peptide chain as desired. See, e.g., Doring et al., Science 292:501 (2001).
A major limitation of the abovementioned strategy is that all sites corresponding to a particular natural amino acid throughout the protein are replaced. The extent of incorporation of the natural and non-native amino acid may also vary because it is difficult to completely deplete the cognate natural amino acid inside the cell. Another limitation is that these strategies make it difficult to study the mutant protein in living cells because the multisite incorporation of analogs often results in toxicity. Finally, this method is applicable in general only to close structural analogs of the common amino acids, again because substitutions must be tolerated at all sites in the genome.
More recently, orthogonal tRNAs and corresponding orthogonal aminoacyl-tRNA synthetases that charge the orthogonal tRNA with the desired non-native amino acid has been used as a strategy to overcome previous limitations. An orthogonal tRNA is a tRNA that base pairs with a codon that is not normally associated with an amino acid such as a stop codon or 4 base pair codon, etc. Importantly, orthogonal components do not cross-react with any of the endogenous tRNAs, aminoacyl-tRNA synthetases, amino acids, or codons in the host organism.
A commonly used orthogonal system for the incorporation of non-native amino acids is the amber suppressor orthogonal tRNA. Using this system, a suppressor tRNA is prepared that recognizes the stop codon UAG and is chemically aminoacylated with a non-native amino acid. Conventional site-directed mutagenesis is used to introduce the stop codon TAG at the site of interest in the protein gene. When the aminoacylated suppressor tRNA and the mutant gene are combined in an in vitro transcription/translation system, the non-native amino acid is incorporated in response to the UAG codon which gives a protein containing the non-native amino acid at the specified position. See, e.g., Sayers et al., Nucleic Acids Res. 16:791-802 (1988). Evidence has shown that the desired non-native amino acid is incorporated at the position specified by the UAG codon and that the non-native amino acid is not incorporated at any other site in the protein. See, e.g., Noren et al., Science 244:182-88 (1989); Ellman et al., Science 255: 197-200 (1992). For additional discussion of orthogonal translation systems that incorporate non-native amino acids, and methods for their production and use, see also Wang and Schultz, Chem. Commun. 1:1-11 (2002); Xie and Schultz, Methods 36:227-38 (2005); Xie and Schultz, Curr. Opinion in Chemical Biology 9:548-554 (2005); Wang et al., Annu Rev. Biophys. Biomol. Struct. 35:225-49 (2006); and Xie and Schultz, Nat. Rev. Mol. Cell. Biol. 7:775-82 (2006).
However, the incorporation of non-native amino acids using orthogonal components suffers from much lower yields because it relies on inherently inefficient suppressor tRNAs competing with termination factors. In addition, the use of orthogonal components for incorporation of non-native amino acids has been restricted to selective incorporation of only a single non-native amino acid per protein at only one of the three nonsense termination codons (the UAG amber stop codon) because of competition at amino acid sense codons from natural amino acids catalyzed by the tRNA charging and proofreading activities of the twenty different aminoacyl-tRNA synthetases, and because attempts to use a second termination codon (UGA) often fails due to read-through by the ribosome. See, e.g., Cload et al., Chem. and Biol. 3:1033-38 (1996).
While some attempts have been made to incorporate non-native amino acids into proteins using tRNAs that recognize sense codons, such attempts have been made using a pure reconstituted in vitro translation system. See Tan et al., Methods 36:279-90 (2005); Forster et al., U.S. Pat. No. 6,977,150. However, such pure reconstituted translation systems require purified translational components, which is impractical outside of the context of research, very expensive, and not shown to be highly efficient.
There exists a need in the art for incorporating non-native amino acids into a growing polypeptide chain, where orthogonal tRNA/aminoacyl-tRNA synthetase pairs can be avoided, where native isoaccepting tRNAs aminoacylated with non-native amino acids recognize sense codons and subsequently incorporate the non-native amino acid into a growing polypeptide chain at a position defined by the sense codon, where numerous non-native amino acids can be incorporated at defined positions, and where a crude cell-free protein synthesis system can be used that avoids the impracticality, expense, and inefficiency of a pure reconstituted in vitro translation system. The invention described herein fulfills these and other needs, as will be apparent upon review of the following disclosure.